1. Field of the Invention
The present invention relates to a digital broadcast receiving apparatus and a method for switching programs in digital broadcasting.
2. Description of the Related Art
In recent years, various activities for standardizing digital broadcasting are being vigorously carried on in many countries. Broadcasting systems called "ATV" (advanced television) and DVB (digital video broadcasting) have seen suggested in the Unites States and in Europe, respectively, and are under research and development for being put into practical use within a couple of years. In Japan, the activities for standardizing digital satellite broadcasting using a communication satellite have also been proceeding. All of these broadcasting systems are being developed based on the MPEG2 standard, which became an international standard in November 1994.
In digital broadcasting, the number of channels is greatly increased as compared with that of current broadcasting. However, the larger the number of channels becomes, the more sophisticated the procedure for selecting a channel or a program is required to be. More specifically, since transport streams corresponding to multiple programs include a large number of individual video and audio streams to be transmitted, information indicating which program should be selected from a plurality of programs or information indicating which packets should be retrieved from the large number of streams is required. Thus, in the MPEG2 standard, various kinds of transmission control information required for receiving a desired program is supposed to be transmitted in the form of a plurality of additional information tables called "PSI (program specific information)" tables. There are the following three kids of PSI tables complying with the MPEG2 standard: a PAT (program association table); a PMT (program map table); and a CAT (conditional access table).
A PAT is a table showing a relationship between a program.sub.-- number and a packet ID (PID) specifying a PMT (such a PID is called a "PMT.sub.-- PID"). A PMT is a table showing a relationship between a program.sub.-- number and the PIDs of the elements, such as video elements, audio elements and clock references, composing the program (such PIDs are called "elementary PIDs"). In this case, a PID is a number for identifying a packet and is also called a "packet identifier". The PIDs assigned to video packets and audio packets are called "elementary PIDs".
In this way, the necessary information is indirectly described while distributing the information between a PAT and a PMT. This is because if all the information is described in only one table, the size of the table becomes too large so that the capacity of the memory for storing such a large table therein is also disadvantageously increased and it takes a lot of time to access the desired information.
A CAT is used in a chargeable broadcasting system or the like, and is an additional table used by a permitted user for decoding and reproducing scrambled streams or a table showing a relationship between the PIDs and EMM (entitled management message) streams.
Furthermore, though it is not defined in the MPEG2 standard, an NIT (network information table) is defined as a table required for receiving programs in the DVB standard in Europe. The names of networks and various transmission parameters are included in an NIT. An NIT is a table showing a relationship between a program number and transport stream IDs individually corresponding to a plurality of physical channels and like relationships. Based on this table, it is possible to know a correspondence between a program number and the physical channel number through which the program is transmitted (or a transport stream ID).
On the other hand, according to the MPEG2 standard, data, the contents of which are not defined and are to be defined by a user himself, is called "private data". Private data of a section type is specifically called a "private section".
It is noted that there is a detailed description of the terms cited above in ISO/IEC 13818-1, November 1994.
Next, the processing flow executed in a receiving apparatus (currently under development) for changing program numbers will be described with reference to FIG. 17. Note that FIG. 17 shows a case where the propagation path is a satellite.
Assume that a program.sub.-- number=x is now being received and that signals, transmitted from a transponder (or a radio wave repeater) having a transponder number n0 selected from a plurality of transponders provided for a satellite, are now being received (Step n1). During the receiving, various tables including the above-described NITs, PATs and PMTs are received and then stored into a memory.
In this case, it should be noted that a "program number" complying with the MPEG2 standard corresponds to a channel number in conventional analog broadcasting and does not correspond to an individual program such as a news telecast, a sport on-the-spot telecast and the like.
Next, if a viewer changes the program number to select a program.sub.-- number=y (at Step n2), a search is performed to determine whether or not the program.sub.-- number=y is included in the PAT of the transponder n0 which is now used for receiving, or whether or not a program having the program.sub.-- number=y is transmitted from the transponder n0 (at Step n3). In satellite delivering, the number of a transponder individually corresponds to a transport stream ID.
In the case where the program.sub.-- number=y is included in the PAT of the transponder n0, a packet ID of a PMT (or a PMT.sub.-- PID) corresponding to the program.sub.-- number=y is found (at Step n4); elementary PIDs (EL.sub.-- PIDS) corresponding to the program.sub.-- number=y are found on the PMT having the PID (at Step n5); and then video packets, audio packets and program clock references (PCRs) having the EL.sub.-- PIDs are extracted, thereby performing video decoding and audio decoding and reproducing the clocks (at Step n6).
On the other hand, in the case where the program.sub.-- number=y is not included in the PAT of the transponder n0 which is now used for receiving, it is searched for on an NIT with which transponder number (or transport stream ID) the program.sub.-- number=y is transmitted. Assuming that the program.sub.-- number=y is transmitted from a transponder having a transponder number n (at Step n7), the receiving frequency is switched to the frequency of the transponder n for receiving PATs, PMTs and the like (at Step n8) so as to return to Step n4. That is to say, a packet ID of a PMT (or a PMT.sub.-- PID) corresponding to the program.sub.-- number=y is found on the PAT of the transponder n (at Step n4); elementary PIDs (EL.sub.-- PIDs) corresponding to the program.sub.-- number=y are found on the PMT having the PID (at Step n5); and then video packets, audio packets and program clock references (PCRs) having the EL.sub.-- PIDs are extracted, thereby performing video decoding and audio decoding and reproducing the clocks (at Step n6).
The processing flow currently under development for changing program numbers is generally executed in such a manner as described above.
However, in such a conventional method for changing program numbers, the time required for obtaining a packet ID of a PMT (a PMT.sub.-- PID) greatly differs depending upon whether or not the program number to be selected is included in the PAT before the switching or whether or not the above-described process steps n7 and n8 are required to be performed. Accordingly, such a method adversely gives some restlessness to a viewer.